Cardiac output (CO) is defined generally as the volume of blood pumped through the heart per unit time, and is an important factor in monitoring the output of blood in heart failure patients. In intensive care units and surgical sites, for example, cardiac output is sometimes used in conjunction with other hemodynamic parameters to monitor the state of the patient's heart and circulatory system both during and after surgery. In surgical procedures where coronary artery bypass grafting or heart valve replacement is to be performed, for example, hemodynamic parameters such as stroke volume, heart rate, and cardiac output are sometimes used to assess heart performance both during and after the procedure. The monitoring of hemodynamic parameters can also be used for optimizing therapy provided to the patient via a pacemaker or cardiac defibrillator, and in detecting and assessing long term heart disease in certain, at-risk individuals. In some cases, hemodynamic parameters such as cardiac output can also be used to determine if a patient is dehydrating as a consequence of too many diuretics.
A variety of different techniques have been developed for measuring cardiac output within a patient. In one technique known as the Fick method, a measurement of the concentration of oxygen in the pulmonary artery, a peripheral artery, as well as respiratory oxygen are used to estimate cardiac output of the heart. In another technique, cardiac output is estimated using Doppler or duplex ultrasound techniques by measuring the flow velocity and the dimensions across the aortic and/or pulmonic annulus, or alternatively, across the aorta. The flow profile from these measurements provides the stroke volume, which is then multiplied by the heart rate in order to determine cardiac output. In yet another technique, a measurement procedure performed during catheterization uses thermodilution to estimate cardiac output by injecting a bolus of cold saline solution into the right ventricle, and then measuring the resulting temperature curve as the solution flows through the main pulmonary artery.
A number of different pulsed pressure (PP) algorithms have also been employed to estimate cardiac output by analyzing the shape of the blood pressure waveform at a given location within the body. In some systems, for example, a catheter or other device may be positioned at a location within the body such as the aorta or left radial artery for sensing physiological parameters such as blood pressure. These systems typically analyze only the left side, or systemic blood pressure, however, which are typically more accessible for acute or semi-acute applications. Left-side pulsed pressure algorithms used for estimating cardiac output are not directly applicable to right side pressure waveforms, and are therefore often ineffective in extracting many hemodynamic parameters. For example, the diastolic waveform derived using right side pressure measurements does not behave as a decaying exponential, and is instead dominated by wave reflections and artifacts. As a result, it is often difficult to extract vascular parameters employed in pulsed pressure algorithms for computing hemodynamic parameters such as cardiac output from right-side pressure waveforms.